It has been known for some time that when sugar alcohols are dehydrated by means of acid catalysts they form one or more anhydro sugar alcohols. In particular, when the hexitols, sorbitol and mannitol, are dehydrated, they give mixtures of mono and dianhydrohexitols. Such compounds are useful as polyol components in making polyester and polyurethane polymers. However, previous processes for the purification of such compounds have been so costly that they have found comparatively little use in the making of polymers.
When sugar alcohols are dehydrated, the reaction mixtures frequently contain, in addition to the desired products, various degradation products and the products of secondary reactions, as well as unconverted starting materials, solvents and catalysts or their neutralization products. Although some impurities may be present in only small amounts, such impurities are detrimental to their use in specific reactions, particularly in the production of polyesters.
The first step in purification of anhydro sugar alcohols is usually accomplished by methods used in sugar chemistry which involve treatment of their solutions with such materials as activated carbon and ion-exchange resins. The anhydro sugar alcohols are then isolated from the partially purified reaction mixture by multi-step processes. These commonly include a preliminary separation by means of chromatography, and extraction or fractional distillation under reduced pressure. Further purification is achieved by fractional crystallization of the material from solvents such as ethyl acetate or lower aliphatic alcohols.
One process which has been disclosed for the purification of anhydro sugar alcohols involves the removal of impurities by complexing them with the borate ion before distillation (U.S. Pat. No. 3,160,641). Another process employs converting the crude dehydration mixture into acetal derivatives by means of an aldehyde or ketone. The acetal derivatives are then separated and the purified materials are regenerated from their acetals (U.S. Pat. No. 3,484,459).
Even these complicated and expensive purification procedures frequently fail to give a product of sufficient purity for a specific use, or the yields of purified product are so low that the process is not economical. In particular, residual amounts of crystallization solvents, such as ethyl acetate, make them unsuitable for certain uses, such as the manufacture of polyesters.